Thermochromic technologies are commonly employed to monitor temperature changes because of their easy visual detection. Thermochromic technologies generally fall into two categories: reversible and irreversible. Reversible thermochromic technologies include encapsulated thermochromic dye-based systems. Thermochromic dyes are robust temperature indicating systems that are used in many applications. These dyes can be contained in printing inks and extruded plastic parts. Most thermochromic dye-based systems include microencapsulated mixtures of chemicals that are highly colored in the cool state. When the chemical are heated, a reversible reaction occurs that bleaches the system, rendering it colorless. The color returns when the system is cooled. Another reversible thermochromic technology is based on liquid crystals (LC). A LC has strong light reflection at a temperature between a low (crystalline) and a high (isotropic) temperature state. As the temperature of the LC is raised from the low temperature to the high temperature, the color of the liquid crystal goes from the background black, through the spectral colors, and back to black. Upon cooling, this color sequence is reversed.
In many applications, it is highly desirable to have a permanent record when the temperature of an object exceeds a threshold temperature. For example, it may be useful to know if a system has overheated, particularly if the overheating was temporary. During the operation of electrical equipment, overheating caused by loose connections or spikes of current/voltage is a common cause of failure. Equipment which can experience overheating includes cable trays, switchboards, and motor control centers. If overheating is caught early, serious damage (fire, explosion, machine shut down, etc.) to equipment and personal injury can be avoided. However, without continuous monitoring, reversible temperature indicating technology can fail to alert personnel to a temporary overheating. Therefore, an irreversible temperature-indicating method is desirable.
Some irreversible temperature indicating methods use an indicator substrate below a solid substance. In these methods, the indicator substrate becomes visible when the solid substance becomes transparent upon melting.
U.S. Pat. No. 5,152,611 describes an article coated with an opaque composition composed of an organic polymer and a bonding agent. When the melting point of the composition is reached, the material melts and becomes transparent or translucent and the color of the article shows through.
U.S. Pat. No. 5,622,137 describes a temperature sensor that includes a substrate having a colored patch obscured by a thermochromic material. When the material is exposed to a predetermined temperature, it changes to a relatively transparent color whereby the colored patch on the substrate is rendered visible through the thermochormic material.
U.S. Pat. No. 5,779,364 describes a temperature indicator that utilizes waxes, low-temperature melt metals or polymers attached to a support member that will melt to reveal a visible indicia on the support surface, thereby serving as an indicator that the product to which the indicator has been attached has been heated beyond a predetermined temperature.
U.S. Pat. No. 7,063,041 describes a temperature sensitive label having a waxed layer that comprises a powdered petroleum wax formed on the face of a colored paper and covered with a transparent film. When the temperature of an apparatus to which the label is affixed reaches a preset temperature the wax melts and the wax layer becomes translucent.
Unfortunately, achieving a dramatic contrast between the opacity of the solid substances and the transparency of the melted substances in existing temperature indicating films has proven challenging. In addition, known temperature indicating films may not be sufficiently flexible or elastic to be stretched and applied to irregular surfaces.